1. Field of the Invention
The present invention relates to a cooling system for a vehicle, in particular to a cooling system controller for a vehicle which is capable of improving cooling efficiency of a radiator by maintaining a coolant temperature as high as possible, and minimizing the required power of a cooling fan by maintaining the cooling fan speed as low as possible.
2. Description of the Prior Art
A cooling system for a vehicle is for cooling the heat generated by components such as an engine and a transmission, and maintaining the optimum temperature of the components. The temperature of combustion gas inside of the cylinder of the engine of a vehicle rises above 2,000xc2x0 C. when the vehicle operates, and the temperature of a cylinder, a cylinder head and a piston valve also rises according to it. Unless this heat is removed, it may cause problems such as a mechanical trouble and a life span lowering problem due to degradation of the strength of structural parts, a power decrease due to deterioration of combustion, and an abnormal abrasion problem or a seizing problem of moving parts due to oil film damage or oil degeneration on mechanical friction portions caused by the high temperature. Accordingly, a cooling system for cooling the components is required. Meanwhile, if the coolant temperature becomes too low due to overcooling, the exhaust gas state becomes worse and the efficiency of the engine is lowered, and accordingly the cooling system is required to have a function for maintaining the temperature of the heat-generating device appropriately so as not to be too high or low.
There are two cooling methods for keeping the temperature of the heat-generating device appropriately. One is an air i.e., gas cooling method which cools the heat-generating device by using the ambient air as the heat transfer medium, and the other is a liquid cooling method which cools the heat-generating device by circulating a liquid coolant and then exhausting heat from the liquid coolant to the ambient. The latter shows better cooling efficiency than the former, and accordingly the liquid cooling method is used in general.
In the cooling system for an engine adapting the liquid cooling method, in order to cool the heat-generating device, a low temperature water-based coolant is flowed through a water jacket installed around or in an outer part of the heat-generating device such as the engine in order to cool the heat-generating device to the appropriate temperature, and while cooling the heat-generating device, the low temperature coolant becomes heated to a high temperature by heat exchange from the heat-generating device, and the high temperature coolant is flowed to a radiator and become low temperature coolant by transferring its heat to the ambient air at the radiator, and the low temperature coolant is flowed again into the water jacket of the heat-generating device by a pump in order to cool the heat-generating device, and the above-described process is performed continuously. Herein, a cooling fan is installed on one side of the radiator in order to force the cooling air for cooling the liquid coolant flow through the radiator, and the heat exchange between the cooling air and the liquid coolant is performed by the radiator.
As depicted in FIG. 1, the required power HP needed for driving the rotating operation of the cooling fan and the cooling air flow rate Q provided to the radiator by the rotating operation of the cooling fan have a nonlinear relation to the cooling fan speed n. In particular, the required power is considered to be proportional to the cube of the cooling fan speed. Accordingly, when the cooling fan speed is lowered, the required power decreases greatly. Meanwhile, the heat transfer efficiency or cooling efficiency heightens in proportion to the higher temperature of the coolant at the inlet of the radiator flowed from the heat-generating device, because the heat transfer from the radiator to the cooling air can be quickly performed when the temperature difference between the hot coolant and the cooling air is large, and accordingly, the higher the coolant temperature rises, the smaller the cooling air flow rate required to perform the cooling sufficiently, and the cooling fan speed can be lowered accordingly. As described above, maintaining the temperature of the coolant at a high state enables the decreasing of the cooling fan speed and the required power for the cooling the fan.
As depicted in FIG. 1, by raising the coolant temperature at the radiator, the cooling fan speed can be lowered from n1 to n2 and the cooling air flow rate is decreased from Q1 to Q2. Although the cooling effect is the same, the required cooling power for driving the cooling fan can be different in accordance with a control method of the cooling fan.
There are two control methods for controlling the cooling fan. One method is an ON/OFF control method as the simplest method which controls the cooling fan to be either ON/OFF, and the other method is to adjust the average speed of the cooling fan about time to be n1 by controlling the ON/OFF state of the cooling fan repeatedly. The average speed increases in proportion to the time duration of the ON state of the cooling fan, and the required power increases in proportion to average speed. It can be defined by Equation 1.                    HPd2        =                              (                                          n                2                                            n                1                                      )                    ⁢                      xe2x80x83                    ⁢          HPd1                                    [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
As defined in Equation 1, the required power decreases in proportion to the variation of flow rate of the cooling air. However, the required power is in direct 1:1 proportion to the fan speed. Accordingly, the advantage of the higher coolant temperature is not so great because the decrease in the required power from HPd1 to HPd2 is small.
The other method is a stepless speed control method which is capable of reducing the required power more in comparison with the ON/OFF control method with the equal flow rate of the cooling air, and accordingly it is a more efficient method than the ON/OFF control method. Applying the example described above, the cooling fan speed can be lowered from n1 to n2 when the flow rate of the cooling air is decreased from Q1 to Q2. The required power decreases from HPc1 to HPc2 due to the increase in the coolant temperature, namely, the change of the required power is in proportion to the cube of the cooling fan speed. It can be defined as in the following Equation 2.                     HPc2        =                                            (                                                n                  2                                                  n                  1                                            )                        3                    ⁢                      xe2x80x83                    ⁢          HPc1                                    [                  Equation          ⁢                      xe2x80x83                    ⁢          2                ]            
In this case, the required power for the cooling fan is very sensitive to the variation in the coolant temperature and decreases on a large scale with a small change in the flow rate of the cooling air. Therefore, it is clear that the stepless speed control method is more efficient, and the coolant temperature in the radiator should be kept as high as possible in order to obtain the highest efficiency from the cooling system.
In the conventional cooling control methods, the coolant temperature is maintained lower than the optimum coolant temperature in order to prevent overheating, and the cooling fan speed is controlled by various methods in accordance with the coolant temperature. As depicted in FIG.2, a multilevel speed control method has been dominant among the conventional methods. In this method, the cooling fan speed is determined in accordance with the coolant temperature by referring to a look-up table in a digital control unit, or is controlled by a thermostatic switch in a sequence control unit. The control of the cooling fan speed is divided into about four steps. As depicted in FIG.2, the coolant temperature is usually controlled in a much lower range than the optimum temperature because of the large variation between steps. This type of control method is described in U.S. Pat. Nos. 4,955,431, 5,018,484, 5,133,302, and Korean Patent 0185443 and Korean Laid-Open Patent Publication No. 1998-053909.
Meanwhile, in the conventional stepless variable speed control method, the cooling fan speed is controlled in proportion to the operation temperature. Although this method enables to get the coolant temperature closer to the optimum temperature than the multilevel speed control method, the variation in the coolant temperature is still large, and accordingly the temperature of the coolant has to be kept at least 5xc2x0xcx9c10xc2x0 lower than the optimum temperature. The conventional stepless variable speed control method in which the fan speed is controlled proportionally to the coolant temperature is described in the U.S. Pat. No. 5,609,125 and Koran Laid-Open Patent Publication No. 1998-053078.
As described above, the required power for cooling can be minimized by maintaining the cooling fan speed as low as possible and maintaining the coolant temperature as high as possible, but this type of control method has not been embodied in the conventional cooling system because of the following problems.
The first problem is a time delay phenomenon of the cooling system. When a vehicle operates, the states of the engine and transmission are rapidly changed in accordance with the operating environment. When the heating rate of the heat-generating device varies widely due to the abrupt change in the operating environment, a certain time is required to cool the heated coolant to the optimum temperature. In other words, a time delay occurs. As depicted in FIG. 3, the cooling fan starts to work at the time t1 and stops at the time t3, but the coolant temperature does not begin to lower instantly, but rises until the time t2 after a certain time from the time t1, reaches the highest temperature at the time t2, and reaches the lowest temperature at the time t4 after a certain time from the time t3. Due to this time delay, although the cooling fan quickly operates, the coolant can become overheated, because the coolant temperature continuously rises for a certain time without lowering instantly, and accordingly the coolant temperature has to be kept lower than the optimum temperature in the conventional method in view of this problem.
The second problem is the uncertainty of the operating environment. The heat transfer efficiency of the radiator changes according to the ambient atmospheric temperature, and even when the heating rate is constant, the cooling fan speed may need to be changed in accordance with the ambient atmospheric temperature. In particular, the change in heat transfer characteristics of the heat exchanger device caused by the ambient atmospheric humidity and pressure, abrupt change in the heating rate due to vehicle braking, engine braking and operation of an air conditioner, and operating efficiency of the cooling fan during the vehicle operation are the major uncertainties of the operating environment. As described above, the uncertainty of the operating environment is a big problem for determining the cooling fan speed on the basis of the coolant temperature.
The third problem is the variation in the operation circumstances due to deterioration of the heat-generating device and the radiator. When the heat-generating device such as the engine or transmission deteriorates with age, the operating efficiency thereof becomes reduced and the heating rate increases, and when the radiator deteriorates due to the passage of time, the heat transfer efficiency is lowered. In this case, the coolant temperature has to be set much lower than the optimum temperature in consideration of the deterioration of the radiator.
Because of the above-mentioned problems, the conventional cooling system has to maintain the coolant at a lower than optimum temperature for obtaining adequate cooling efficiency.
An object of the present invention is to provide a cooling system for a vehicle which is capable of improving the heat transfer efficiency of a radiator and minimizing the power required for cooling by using a high coolant temperature for obtaining the optimum cooling efficiency and preventing an overheating problem due to time delay characteristics of a cooling system.
Another object of the present invention is to provide a cooling system for a vehicle which is capable of improving the heat transfer efficiency of a radiator and minimizing the power required for cooling by using a high coolant temperature for obtaining the optimum cooling efficiency by considering operational circumstances of the vehicle in speed control of the cooling fan.
A further object of the present invention is to provide a cooling system for a vehicle which is capable of improving the heat transfer efficiency of a radiator and minimizing the power required for cooling by using a high coolant temperature for obtaining the optimum cooling efficiency by considering the deterioration of a heat generation device and radiator caused by the passage of time in speed control of the cooling fan.